1. Field of the Invention
Embodiments of the invention relate generally to semiconductor devices and related methods of fabrication. More particularly, embodiments of the invention relation to semiconductor devices comprising a photo-detecting device adapted to convert optical signals into electrical signals and related methods of formation.
This application claims priority to Korean Patent Application 2004-108791 filed on Dec. 20, 2004, the subject matter of which is hereby incorporated by reference.
2. Description of Related Art
Photo-detecting devices are common constituents of contemporary semiconductor devices adapted to detect externally provided optical signals and converting these to corresponding electrical signals. For example, photo-detecting devices have been used as a driver (or optical pick-up component) adapted to read data recorded on optical storage media, such as compact disc (CD) or digital versatile disc (DVD). Recent product design trends are demanding better access speeds from drivers, and in general photo-detecting devices face increasing pressure to operate at ever faster operating speeds.
Conventional photo-detecting devices typically include a photodiode serving as a light receiving portion of the device. A typical conventional photodiode will now be described below with reference to FIGS. 1 through 4.
FIG. 1 is a plan view of a photodiode included within a conventional photo-detecting device. FIG. 2 is a cross-sectional view taken along a line I-I′ of FIG. 1.
Referring to FIGS. 1 and 2, a buried p-type (e.g. doped with p-type impurities) doping layer 3 is disposed on a silicon substrate 1. (Throughout hit description the term “on” in this context means either one layer, device, and/or region “directly on” another layer, device and/or region, or one layer, device, and/or region on another with intervening layers, regions and/or devices present). A p-type silicon epitaxial layer 5 and an n-type silicon epitaxial layer 7 are then sequentially stacked on buried doping layer 3.
An isolation doping layer 8 defines an n-type silicon body region 7′ within n-type epitaxial layer 7. P-type isolation doping layer 8 extends downward through n-type epitaxial layer 7 to electrically connect p-type epitaxial layer 5. N-type body region 7′ is thus isolated by isolation doping layer 8 and n-type epitaxial layer 5.
A p-type buffer doping layer 6 is disposed at the portion of p-type silicon epitaxial layer 5 contacting isolation doping layer 8. The doping concentration for buffer doping layer 6 is higher than that of p-type epitaxial layer 5.
A heavily doped n-type layer 10, having a doping concentration higher than that of n-type body region 7′, is formed on an upper portion of n-type body region 7′. Heavily doped n-type layer 10 and n-type body region 7′ correspond to an “n-region” of the photodiode being formed, and p-type epitaxial layer 5 and buried doping layer 3 disposed below the n-region correspond to a “p-region” of the photodiode.
An interlayer oxide layer 12 is formed on the resulting structure. An opening is formed in interlayer oxide layer 12 to expose heavily doped n-type layer 12. However, interlayer oxide layer 12 covers an edge portion of heavily doped n-type layer 10. First and second interconnections 15a and 15b are formed through interlayer oxide layer 12. First interconnection 15a is electrically connected to isolation doping layer 8 through interlayer oxide layer 12. Accordingly, first interconnection 15a is electrically connected with the p-region of the photodiode through isolation doping layer 8 and buffer doping layer 6. Second interconnection 15b is connected to the edge portion of heavily doped n-type layer 10 through interlayer oxide layer 12.
An operating method of this exemplary, conventional photodiode structure now will be described in the context of the energy band diagram illustrated in FIG. 3. In FIG. 3, reference numerals “a”, “b” and “c” denote respectively the n-type neutral region, a depletion region, and a p-type neutral region, of the photodiode.
Referring to FIGS. 1-3, a reverse bias may be applied to the photodiode at a standby state. The depletion region “b” may include a majority of n-type body region 7′, as well as a majority of p-type epitaxial layer 5 below n-type body region 7′.
The photodiode is divided into the n-type neutral region “a”, the depletion region “b”, and the p-type neutral region “c” by a PN junction. The n-type and the p-type neutral regions “a” and “c” are placed in a neutral state under conditions wherein electric fields are not established. An internal electric field 20 is established in depletion region “b”.
In the standby state, an external optical signal is applied to the photodiode through heavily n-type doped layer 10. The applied optical signal generates electron-hole pairs (EHPs) in the photodiode. The generated electrons migrate along a conductivity band Ec and the generated holes migrate along a valence band Ev, allowing signal current to flow between first and second interconnections 15a and 15b. 
In the conventional photodiode, the electrons and holes generated in depletion region “b” in response to the applied optical signal are accelerated by internal electric fields 20 in depletion region “b”. In contrast, electrons and holes generated in depletion regions “a” and “c” migrate by means of diffusion. This migration path for the electrons and holes increases their migration time, so that the response characteristics of the photodiode (i.e., a reactive speed with respect to the optical signal) are degraded.
More particularly, the optical signal is applied with greatest intensity to the surface of n-type neutral region “a” (i.e., a surface of heavily n-type layer 10). The greater the depth of this intensity from the surface of n-type neutral region “a”, the stronger the intensity of the optical signal. Therefore, diffusion migration of the electrons and holes generated in n-type neutral region “a” is a major factor in the degradation of response characteristics for the photodiode.
Exemplary response characteristics for the conventional photodiode now will be described with reference to FIG. 4. In FIG. 4, the x-axis represents time required for applying an optical signal, and the y-axis represents the amount of signal current generated by the optical signal. Additionally, a reference numeral “t” indicates an instance in time at which the optical signal is no longer applied to the photodiode.
Referring to FIGS. 1-4, when an optical signal having a determined intensity is applied to the photodiode, electron-hole pairs are generated mainly in n-type neutral regions “a” and depletion regions “b”. The generated electrons and holes migrate due to diffusion and internal electric fields 20, allowing signal current to flow between interconnections 15a and 15b. For a given optical signal intensity, signal current will flow at a corresponding rate.
From time “t” forward when the optical signal is no longer applied to the photodiode, the amount of the signal current flowing from the photodiode decreases rapidly. Ideally, this signal current would stop immediately at time “t”. However, some diminishing amount of signal current, or tail current 25, continues to flow after time “t” due to the migration time lag of electrons and holes generated in the photodiode.
At time “t”, the holes generated in depletion region “b” migrate more quickly under the influence of internal field 20 while the holes generated in n-type neutral region “a” migrate to depletion region “b” via diffusion effects. Therefore, the migration time for holes generated in n-type neutral region tends to increase the length of tail current 25.
Unfortunately, as the tail current 25 increases in length, the response characteristics of the photodiode are degraded, thereby decreasing the overall operating speed of the photodiode. Moreover, tail current 25 may actually result in increased noise distorting the output signal characteristics of the photodiode.